I have a 1990 560SEC, with - original - R12 aircon system. It has the rotary dial controls, not the pushbutton ones.
Until recently the aircon worked fine, but the car has been garaged over the winter. I've just started running it again, and the aircon doesn't work. The compressor clutch isn't engaging.
As there are few cars with aircon where I live (the north of Scotland) and even fewer aircon service technicians, I need to do some initial diagnosis if I can. What should I check, and how, to establish why the compressor clutch isn't engaging? I assume that the most likely explanations are (a) some electrical problem - switch, fuse, or wiring; (b) low gas pressure - so a safety cutout is preventing the clutch from activating; or (c) some mechanical fault with the compressor-clutch itself.
Apart from checking the fuses, how can I test and in what order for possible electrical problems? And is there any way I can check - I have no specialist tools or gauges - on whether the refrigerant is low? I desperately want to retain and preserve the R12 setup if I can, so I'm hoping that the problem is not one which requires complete conversion of the system to R134a.
One other possible clue (but it might be a red herring...) . There are traces of an oily leak on the ground under the car in the area (front left) under the aircon compressor and the power steering and other hydraulic reservoirs. I had assumed that this was indeed most likely to be power steering fluid. But could it be aircon related? I had always assumed that any leaking refrigerant would simply vaporise and disappear into the atmosphere (hence the ozone-layer problem!)
Any advice on basic troubleshooting using essentially eyes, hands screwdriver and a multimeter would be appreciated. Links to tech advice sites, or even better, some pictures, would be even better!
Leaking refrigerant will carry some oil with it but not a lot. It leaves an oily residue rather than liquid oil.
Most likely your car is low on freon. You can jumper the low pressure lockout switch on the receiver drier or you can unplug the compressor and apply 12V. Lots of foam means it's low. Nothing means it's critically low - disconnect! I wouldn't worry about R134a in Scotland. R134a will get real cold there since you do not need maximum system capacity. It's when you expect it to move as much heat as R12 in places like Saudi Arabia that it falls flat.
I had to troubleshoot a nonfunctional compressor clutch on my '83 300SD when I first got it. First, check the fuses and A/C relay by substituting a good known relay for it (there are several identical relays in the box so this should be no problem). There are two electrical switches in series in the a/C compressor clutch control circuit: one is in the climate control unit, it turns A/C on and off. The other one is a thermostat switch with a probe going into the evaporator. This was the culprit in my case. I simply jumpered the switch leads and the compressor clutch turned on. This switch is located under the dash on the driver's side, closer to the center console. You need to remove the lower panel under the dash to get to it.
The system must have enough refrigerant for the compressor clutch to turn on. Also, you need to check the clutch itself. Supply a 12 Volt battery power directly to the clutch terminals and see if it will engage. Also, measure the resistance of the clutch windings. It should be about 5 ohms. Infinite resistance means a burned out clutch.
First, check the fuses and A/C relay by substituting a good known relay for it (there are several identical relays in the box so this should be no problem).
Are you talking about the fuse box? If so, I know of no relay in there for the compressor. Two of them are for the aux fan, the rest are for power windows, seats, air injection system, etc. At least from what I could gather by looking at the electrical schematics in the CD this weekend.
A quick update - it's not the fuse. Fuse #5 in the box covers the entire aircon system and various other circuits as well, and it's good. And there's power to the switch in the dash, because the little red light illuminates when I switch the a/c on.
I'm ready to check and/or substitute the relay, once it's clear where I find it...
As for testing the clutch itself, i'm confused. If you apply 12v power direct to the clutch terminals, but the refrigerant is low, will the clutch engage? And/or will such a test do any damage if the freon is indeed low?
And by the way, my concern about R134a is not its absolute cooling efficiency, but the headache and expense of changing all the tubes, seals, drier, etc etc etc.
There's a Klima Relay in-between the two firewalls, closer to the brake booster/master cylinder on the driver's side (US), not sure what they did in the UK. I believe it says Klima on it. You can jumper pins 5 & 7 when you take the relay out and see if the a/c compressor engages.
A quick update - and it narrows down the problem, though I'm still thinking about how to solve it. Found a local a/c technician who had all the gauges, kit and fittings to test an R12 system. And guess what - virtually no refrigerant. So that (rather than an electrical fault) was the reason the compressor clutch wasn't engaging.
Did a pressure test with nitrogen, which confirmed there was a TINY leak somewhere (the pressure dropped very very slowly, so it must be like a pinhole or smaller). Impossible to see or hear it.
Now I'm stuck.. The logical further test would be to recharge the system and add some dye and - maybe after a few days or a week - the dye would reveal the site of the leak. But here in UK it's impossible to get R12 even to recharge for such a test. So I seem to have no option but to convert to R134a (drain compressor, get new drier, hose fittings, seals etc). Yet it seems foolish to do that without finding and solving the leak - especially if it's a hose or hose-joint - first. Catch 22!
Thanks... Can I just ask what you mean by "the front seal" - especially if it's a common/known failing? Is this a seal within the compressor? Or a seal at a hose-joint leading into the compressor? As my system has such a tiny leak - but there's circumstantial evidence that it may be somewhere around the compressor area because of the slight oily stain on the garage floor under there - I need all the help I can get in tracing it!
Linear Mode
